Polycarbonates are well known thermoplastic materials which due to their many advantageous physical and mechanical properties find use as thermoplastic engineering materials in many commercial and industrial applications. The polycarbonates exhibit, for example, excellent properties of toughness, flexibility, impact resistance, and high heat distortion temperatures. The polycarbonates and their preparation are disclosed, inter-alia, in U.S. Pat. Nos. 2,964,974, 2,999,835, 3,028,365, 3,334,154, 3,275,601 and 3,915,926.
However, these polycarbonates generally suffer from two disadvantages. The first disadvantage is the low critical thickness values of polycarbonates, i.e. the thickness at which a discontinuity in Izod impact values occurs. These low critical thickness values tend to limit wall thickness of molded polycarbonate to a thickness below the critical thickness. Polycarbonates exhibit notched Izod impact values which are dependent on the thickness of the polycarbonates. Thus, for example, while typical notched Izod impact values of a one-eighth inch thick polycarbonate test specimen are generally in the range of about 16 foot pounds per inch, typical notched Izod impact values for a one-fourth inch thick polycarbonate test specimen are generally in the range of about 2 foot pounds per inch. The high Izod values of the one-eighth inch thick polycarbonate tests specimens are due to the fact that these specimens are thinner than the critical thicknes of the polymer and, therefore, upon impact a hinged or ductile break occurs. The low Izod impact values of the one-fourth inch thick polycarbonate test specimens are due to the fact that these specimens exceed the critical thickness of the polymer and, therefor, upon impact a clean or brittle break occurs.
The second disadvantage of polycarbonates is that they are somewhat flammable. Thus, polycarbonates are generally somewhat unsuitable for some applications where high temperatures and exposure to open flame may be encountered. In order to render polycarbonates suitable for use in these environments they must be modified to be rendered fire retardant. One of these modifications involves the inclusion of a halogenated moiety, such as a halogenated diphenol, in the carbonate polymer itself. These halogen containing carbonate copolymers are generally fire retardant. However, the presence of these halogen containing moieties adversely affects the already low critical thickness values of the polycarbonates. Thus, for example, the critical thickness of a carbonate copolymer containing 5 to 6 percent by weight bromine in the form of a brominated diphenol is typical about 130-140 mils.
The prior art, such as for example U.S. Pat. No. 4,043,980, discloses compositions obtained as the reaction product of an aromatic diol, a halogenated phenol, and a carbonic acid coreacted with an aromatic thiodiphenol, which compositions exhibit flame retardancy stated to be the result of synergism between the sulfur and the halogen present in the compositions. It is also stated that such compositions overcome the detrimental critical thickness effect. However, this prior art teaches the necessity of the presence of both a halogen containing moiety and a sulfur containing moiety in the carbonate polymer, and that the resultant flame retardancy of such halogen and sulfur containing copolycarbonates is the result of synergism between the sulfur and the halogen present in the copolycarbonate.
International Application No. WO 82/00468, published Feb. 18, 1982 discloses that polycarbonate compositions can be rendered fire retardant by either admixing with the carbonate polymer a polymer based on a thiodiphenol, or incorporating into the polycarbonate polymer itself a thiodiphenol residue. While these compositions are quite effective and useful in most applications, they suffer from the disadvantage that relatively large amounts of thiodiphenol, typically from about 23-98 mole percent, must be employed to render said compositions flame retardant and non-dripping. Since thiodiphenol is relatively expensive, as compared with dihydric phenols such as bisphenol-A normally used in the preparation of polycarbonates, its use in relatively large amounts places these flame retardant polycarbonate compositions at an economic disadvantage vis-a-vis the non-thiodiphenol containing polycarbonates. Also, in some applications, particularly those where the polycarbonate resin is required to exhibit properties of substantially sulfur-free bisphenol based polycarbonates, such as for example a bisphenol-A based polycarbonate, or where the presence of relatively large amounts of sulfur would be disadvantageous, such large amounts of thiodiphenol are undesirable. In these applications the prior art thiodiphenol containing polycarbonates could not be used satisfactorily.
It would thus be very advantageous if flame retardant polycarbonate compositions could be provided which are halogen-free and which consequently exhibit the properties of halogen-free polycarbonates, such as good thick section impact strengths. It would also be very advantageous if flame retardant polycarbonate compositions could be provided which not only exhibit he properties of halogen-free polycarbonates, but also exhibit the properties of substantially sulfur-free polycarbonates.
It is, therefore, one of the objects of the instant invention to provide polycarbonates which are flame retardant, are non-dripping, are halogen-free, and consequently exhibit the properties and characteristcs of halogen-free polycarbonates such as good thick section impact.
It is another object of the instant invention to provide polycarbonate compositions which are flame retardant and non-dripping while simultaneously exhibiting the properties and characteristics of substantially sulfur-free, or low sulfur containing, polycarbonate resins.